An ordinary glass lens is either composed of a pair of spherical surfaces or one spherical surface and one flat surface. It is well established that most spherical lenses are typically subject to aberrations. In these types of lenses, as shown in FIG. 7, parallel light rays 52 strike the spherical lens 51 and pass through it. The transmitted light rays 53, however, do not typically converge on a single point along the axis of the light beam, but are instead scattered in various directions depending on their respective angles of incidence. FIG. 8 shows a curve expressing the usual wavefront aberrations of the spherical lens of FIG. 7. The vertical axis h shows the position of incidence, and the horizontal axis shows the phase shift.
To avoid the above problems, non-spherical lenses are sometimes employed which are free of aberrations. However, the processes used to produce a non-spherical glass lens exhibiting such qualities such as non-spherical polishing, are complex which makes such lenses difficult to mass-produce and extremely costly.
It has been found that non-spherical plastic lenses can be mass-produced through a simple process of pouring an optical resin into a mold. This process makes it possible to mass-produce non-spherical lenses at a comparatively low cost. However, the plastic lenses have other drawbacks. When exposed to heat, the plastic lenses will expand more than comparable glass lenses. This results in large variations in quality due to temperature variations. Furthermore, plastic lenses suffer a great deal of shrinkage during molding, with the resulting effect that mass-produced lenses often do not accurately reflect the shapes of their respective molds. Such lenses usually exhibit a profound degree of distortion.